Processes and systems for handling knots in a chemical pulping process

ABSTRACT

Processes and systems are provided for cooking wood chips containing knots in a digester to produce a chemical pulp. Specifically, a low pressure slurry of comminuted cellulosic fibrous material is formed in a vessel operating at essentially atmospheric pressure by mixing therein a liquid and the comminuted cellulosic fibrous material. The low pressure slurry is thereafter supplied to a low pressure inlet of a high pressure transfer device while liquid is removed from the slurry through a low pressure outlet thereof. A high pressure liquid is supplied to the transfer device (e.g., via a slurry pump) so as to form a high pressure slurry of the comminuted cellulosic material which is then discharged from a high pressure outlet of the transfer device. Knots are removed via a knot separator from a slurry of uncooked knot-containing cellulosic material at essentially atmospheric pressure. The removed knots are thereafter transferred to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on, and claims domestic priority benefits under 35 USC §119(e) from, U.S. Provisional Application Serial No. 60/334,963 filed on Dec. 4, 2001, the entire content of which is expressly incorporated hereinto by reference.

FIELD OF THE INVENTION

[0002] The present invention is related generally to chemical wood pulping processes and systems. In particular, the present invention relates to processes and systems for handling of knots in such chemical wood pulping processes.

BACKGROUND OF THE INVENTION

[0003] Screw-type chip meters are typically used to determine the chip feed rate to a digester employed in chemical wood pulping processes. The chip meters therefore are employed to feed uncooked wood chips in a controlled manner to the downstream pre-cooking and cooking operations, for example, chip steaming, impregnation vessels and/or digester vessels. A chip chute typically is employed to convey the uncooked wood chips to such downstream operations.

[0004] Conventionally, knots present in the wood chips are typically removed by screening following at least some cooking. Thus, in many pulp mills, a drainer is provided so as to remove the knots from the pulp, such as the system suggested in U.S. Pat. No. 3,886,035¹. Such knots may then be refined or re-cooked so as to minimize waste and to recover any fiber content that may be present therein. (See, U.S. Pat. No. 4,002,528.) Conventional knot recovery techniques typically involve returning the knots separated from the pulp to the chip bin which requires expensive recycling equipment. Alternatively, knot drainers may be located at the top of the chip bin, but such an arrangement requires excess energy to be used to pump all the filtrate carrying the knots recovered after cooking back to the chip bin for reprocessing.

[0005] Another knot-recycling technique has recently been proposed in U.S. Pat. No. 5,672,245. In this regard, the '245 patent proposes to separate knots by means of a screen downstream of the pulp digesters and then redirect such separated knots, following their treatment in a knot bin with black liquor, to the chip chute upstream of the high pressure feeder.

SUMMARY OF THE INVENTION

[0006] According to the present invention, novel processes and systems are provided for the return of knots removed by conventional means, such as screening in the knotter or screen room, to an essentially atmospheric pressure feed system prior to being cooked in the digester operations associated with a chemical wood pulping process. More specifically, according to the present invention, a flow of uncooked wood chips is metered into a chip chute upstream of the digester operations. The processes and systems of the present invention will therefore allow the return of the knots to the chip feed system by feeding the knots into the chip handling system operating at essentially atmospheric pressure, instead of the conventional technique of returning the knots to a pressurized, higher elevation chip bin. Knots from the knotter or screen room are most preferably returned to the chip feed system through a knot drainer associated operatively with the chip screw.

[0007] The present invention is therefore especially well suited for use with current Lo-Level® Feed Systems commercially available from Andritz Inc. of Glens Falls, N.Y. (See also, U.S. Pat. Nos. 5,476,572; 5,700,355; 5,968,314; 5,766,418; 6,368,453, and 6,436,233.) The use of the present invention should result in lower energy costs by allowing the knot drainer to be placed at a lower elevation (e.g., attached physically to the chip screw instead of the higher elevation top of the chip bin). Additional energy savings are incurred by eliminating the need for the pressurization of the knot stream to the pressure of the conventional feed system.

[0008] These and other aspects and advantages will become more apparent after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0009] Reference will hereinafter be made to the accompanying drawing FIGURE which is depicts one particularly preferred system for treating knots in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0010] As shown in the accompanying drawing FIGURE, one particularly preferred system in accordance with the present invention includes a feed system 10 for introducing, steaming, slurrying and pressurizing comminuted cellulosic fibrous material, for example, hardwood or softwood chips, and feeding the slurry to a continuous digester system (not shown). These systems are disclosed in U.S. Pat. Nos. 5,476,572; 5,622,598; 5,635,025; 5,766,418; and 5,968,314 and are marketed under the trademark LO-LEVEL® by Andritz Inc. of Glens Falls, N.Y.

[0011] Though comminuted cellulosic fibrous material may take many forms, including sawdust; grasses, such as straw or kenaf; agricultural waste, such as bagasse; recycled paper; or sawdust, for the sake of simplicity, the term “chips” will be used when referring to comminuted cellulosic fibrous material; but any and all of the listed materials, and others not listed, may be processed by the present invention. Also, though a continuous digester may be referenced below and in the accompanying FIGURE, it is understood that the present invention as also applicable to feeding several continuous digesters or one or more discontinuous or batch digesters.

[0012] As shown in the FIGURE, chips 13 are introduced to the system, for example, via a conveyor (not shown) from a chip storage facility, for example, a woodyard, via an isolation and metering device 14. For example, the FIGURE illustrates a screw-type isolation device 14 as described in U.S. Pat. No. 5,766,418. The device 14, driven by an electric motor (not shown), introduces the chips to chip retention and streaming vessel 16. Though various types of vessels are known in the art, vessel 16 is preferably a DIAMONDBACK® Steaming vessel as marketed by Andritz Inc. and described in U.S. Pat. Nos. 5,500,083; 5,617,975; 5,628,873; and 4,958,741, or a CHISELBACK™ vessel marketed by Andritz Inc. as described in U.S. Pat. No. 6,199,299. This vessel typically includes a gamma-radiation level-detection system, a regulated vent for discharging gases which accumulate in the vessel and one or more steam introduction conduits 16′. The pressure in the vessel 16 may be slightly below atmospheric pressure or slightly above atmospheric pressure, that is, the pressure in vessel 16 may vary from about −1 to 2 bar gage (that is, about 0 to 3 bar absolute).

[0013] During treatment with steam in vessel 16, the air that is typically present in the chips is displaced by steam and the heating of the chips is initiated. The removal of air from the cavities within the chips permits the more efficient diffusion of cooking chemical into the chip and minimizes the buoyant forces on the chip during subsequent processing.

[0014] The steamed material is discharged from the bottom of the vessel 16 to a metering device 17, for example, a star-type metering device or Chip Meter as sold by Ahlstrom Machinery, though any type of metering device may be used. The metering device 17 is typically driven by an electric motor (not shown) and the speed of rotation of the metering device is typically controlled by operator input to define a set rate of introducing chips to the system. The chips discharged by the metering device 17 are introduced to a vertical conduit or pipe 18, for example, a Chip Tube sold by Ahlstrom Machinery. Cooking chemical and other liquids are typically first introduced to the chips in conduit 18 by means of one or more conduits 19 such that a level of liquid is established in conduit 18 and a slurry of chips and liquid is present in the bottom of conduit 18. This level of liquid is typically monitored and controlled by a level detection device, for example, a gamma-radiation level detection device or a “d-p” cell. The metering device 17 typically does not act as a pressure isolation device, though it may, and the pressure in conduit 18 typically varies from 0 to 2 bar gage (or 1 to 3 bar absolute).

[0015] Conduit 18 discharges the slurry of chips and liquid by means of a radiused section 20 to the inlet of slurry pump 21. Though any slurry pump can be used, pump 21 is preferably a Hidrostal® screw centrifugal pump sold by Wemco Pump of Salt Lake City, Utah or a pump provided by Lawrence Pumps Inc. of Lawrence, Mass. Slurry pump 21, driven by electric motor 21′, pressurizes and transfers the slurry in conduit 18 via conduit 22 to the low pressure inlet 23 of a high pressure transfer device 24. This high pressure transfer device is preferably a High-pressure Feeder as sold by Andritz Inc. High-pressure feeder 24 includes a pocketed rotor mounted in a housing typically having a low-pressure inlet 23, a low-pressure outlet 25, a high-pressure inlet 26 and a high-pressure outlet 27. The low-pressure outlet 25 typically includes a screen plate (not shown) which minimizes the passage of chips out of low-pressure outlet 25 while allowing the liquid in the slurry to pass out outlet 25 to conduit 28, though as disclosed in U.S. Pat. No. 6,199,299, the screen in the low-pressure outlet of feeder 24 may be omitted. The chips which are retained in the feeder by the screen are slurried with high-pressure liquid provided by pump 29, preferably a Top Circulation Pump (TCP) provided by Andritz Inc., to inlet 26 via conduit 30. The slurry is discharged out of high-pressure outlet 27 into conduit 31 and to the digester 32 of digester system 12 at a pressure of between about 5 and 15 bar gage, typically between about 7 to 12 bar gage.

[0016] The digester (not shown) may be a single or multiple-vessel digester and may be a hydraulic or steam-phase digester. The digester may also consist or comprise one or more batch digesters. The cellulose material with added cooking chemical is treated under temperature and pressure in the digester and essentially fully-treated chemical cellulose pulp is discharged into a conduit at the bottom of the digester. Though many types of processes may be performed in the digester, one preferred process is the process described in U.S. Pat. Nos. 5,489,363; 5,536,366; 5,547,012; 5,575,890; 5,620,562; 5,662,775; 5,824,188; 5,849,150; and 5,849,151 and marketed by Andritz Inc. under the trademark LO-SOLIDS®. The process performed in the digester may also be one of the processes disclosed in U.S. Pat. Nos. 5,635,026 or 5,779,856 and marketed under the name EAPC™ cooking by Andritz Inc.

[0017] As shown in the FIGURE, excess liquor in the slurry in conduit 31 at the top of the digester is separated from the slurry by a liquor separator and returned to the feed system 10 by means of conduit 34. The liquid in conduit 34 is pressurized by pump 29, driven by electric motor 29′, and provides the pressurized slurrying liquid introduced to the high-pressure inlet 26 of feeder 24 via conduit 30. Feeder 24 is typically driven by an electric motor (not shown), the speed of which is monitored and controlled.

[0018] As shown in the FIGURE, the liquid discharged from the low-pressure outlet 25 of high-pressure feeding device 24 passes via conduit 28 to a cyclone-type separator 35 which isolates undesirable material and debris, such as sand, stones, etc., from the liquid in conduit 28. Separator 35 is preferably a Sand Separator as sold by Andritz Inc. Liquid having little or no undesirable material or debris is discharged from separator 35 and is passed through a liquor separating device 37 via conduit 36. At least some liquid is removed from the liquid separator 35, which is preferably an Inline Drainer as sold by Andritz Inc., via conduit 38 and sent to vessel 39. Vessel 39 is preferably a Level Tank as sold by Andritz Inc. Liquid is discharged from vessel 39 to conduit 40 and pump 41 and is supplied to the digester as liquor make-up as needed via conduit 42. Pump 41 is preferably a Make-Up Liquor Pump (MLP) as sold by Andritz Inc. The sand separator 35, level tank 36, and in-line drainer 37 can be omitted without interfering with the ultimate function of the feed system 10.

[0019] The liquid discharged from separator 37 into conduit 43 may be supplemented with cooking chemical, for example, kraft white, green, orange (that is, liquid containing polysulfide additives) or black liquor, prior to being introduced to tank 45. Tank 45 is preferably a Liquor Surge Tank as sold by Andritz Inc. and described in U.S. Pat. No. 5,622,598. The cooking chemical may be heated or, preferably, cooled as needed by a heat exchanger (not shown). Some of the liquid in conduit 43 may bypass tank 45 and be introduced via conduit 19 to conduit 18 as described above. Tank 45 communicates with conduit 18 and the inlet of pump 21 via conduits 47 and 20. As disclosed in U.S. Pat. No. 6,368,453, tank 45 may comprise or consist of an integral vessel concentric with conduit 18.

[0020] Important to the present invention, a knot separator 50 is provided so as to supply knots via conduit 52 to the conduit 18 downstream of the metering device 17. Most preferably the knot separator 50 is a Model KW Secondary Knotter from Andritz Inc. The knot separator 50 separates knots from the inlet supply of slurried knots and liquor pumped from a primary knotter or a knot screener introduced via inlet conduit 54. Within the knot separator 50, a vertically oriented screw-type conveyor is driven by motor 55 and lifted into the discharge chute 56 connected to the conduit 52. The separated liquor is transferred via chute 58 to chamber 60 and thereafter discharged via conduit 62. Gas may be vented from the chamber 60 via conduit 64.

[0021] The present invention therefore allows the return of the knots to the chip feed system 10 by feeding the knots into the conduit 18 at under essentially atmospheric conditions instead of the conventional technique of returning the knots to a pressurized, higher elevation chip bin.

[0022] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

What is claimed is:
 1. A process for cooking wood chips containing knots in a digester to produce a chemical pulp, comprising the steps of: (a) forming a low pressure slurry of comminuted cellulosic fibrous material in a vessel operating at essentially atmospheric pressure by mixing therein a liquid and the comminuted cellulosic fibrous material; (b) supplying the slurry to a low pressure inlet of a high pressure transfer device while removing liquid from the slurry through a low pressure outlet thereof; (c) supplying a high pressure liquid to the transfer device and forming a high pressure slurry of the comminuted cellulosic material; (d) discharging the high pressure slurry from a high pressure outlet of the transfer device, (e) removing knots from a slurry of uncooked knot-containing cellulosic material at essentially atmospheric pressure, and thereafter transferring the removed knots to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.
 2. The process of claim 1, comprising using a knot drainer to remove the knots from the slurry of uncooked comminuted cellulosic material.
 3. The process of claim 1, wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein step (e) includes the step of introducing the removed knots to the low pressure slurry at the chip chute.
 4. The process of claim 1, comprising recirculating at least a portion of the liquid removed from the slurry at the low pressure outlet of the transfer device to the vessel.
 5. The process of claim 4, wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein step (e) includes the step of introducing the removed knots to the low pressure slurry at the chip chute.
 6. A system for producing a chemical pulp from a slurry of knot-containing cellulosic material, comprising: (a) a digester for pressurized cooking the knot-containing cellulosic material; (b) a vessel operating at essentially atmospheric pressure for forming a low pressure slurry of comminuted cellulosic fibrous material; (b) a high pressure transfer device which includes a low pressure inlet, a low pressure outlet, a high pressure inlet and a high pressure outlet, wherein the low pressure inlet receives the low pressure slurry from the vessel and the low pressure outlet allows liquid to be removed from the slurry; (c) a slurry pump for supplying a high pressure liquid to the transfer device through the high pressure inlet and thereby form high pressure slurry of the comminuted cellulosic material which is discharged through the high pressure outlet and supplied to the digester; (e) a knot separator for removing knots from a slurry of uncooked knot-containing cellulosic material at essentially atmospheric pressure, and thereafter transferring the removed knots to the vessel so that the knots become part of the low pressure slurry supplied to the low pressure inlet of the transfer device.
 7. The system of claim 6, wherein said knot separator includes a knot drainer to remove the knots from the slurry of uncooked comminuted cellulosic material.
 8. The system of claim 6, wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein step (e) includes the step of introducing the removed knots to the low pressure slurry at the chip chute.
 9. The system of claim 6, comprising a recirculation conduit for recirculating at least a portion of the liquid removed from the slurry at the low pressure outlet of the transfer device to the vessel.
 10. The process of claim 9, wherein the vessel includes a chip chute, and a metering device which discharges a metered amount of the comminuted cellulosic material to the chip chute, and wherein the knots removed by said knot separator are introduced to the low pressure slurry at the chip chute. 